Acoustic shotgun system

ABSTRACT

A high velocity acoustic signal producing underwater shotgun system for dispersing a plurality of relatively small supercavitating projectiles over a wide spatial field at long range using the dynamics of cavity collapse for better target localization in underwater mine clearance. A typical supercavitating projectile design is enhanced to produce a two-staged projectile in order to accomplish this innovation. The first stage of the two stage design allows for the long range firing underwater typical of a supercavitating projectile while the second stage permits the coverage of a wide area with a plurality of small supercavitating projectiles just as the first stage projectile reaches its fixed range. A distinctive feature of the radiated noise from a supercavitating projectile contacting a solid object is used in conjunction with the two stage projectile design to provide a system for underwater mine clearance verification. The distinctive noise signal may also be used in conjunction with an underwater targeting system to help identify, localize and track targets as well.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

CROSS REFERENCE TO OTHER PATENT APPLICATIONS

None.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a high velocity acoustic shotgun systemand more particularly to a method and apparatus for dispersing aplurality of relatively small, supercavitating projectiles in the waterover a wide spatial field at long ranges from an underwater gun orsurface gun.

(2) Description of the Prior Art

One major technical challenge related to employing supercavitatingprojectiles against targets is the need to accurately determine thelocation of a target object when it is positioned at a substantialdistance from the launch device. Even small errors in the target bearingsolution will result in a miss of the object. To overcome this knownproblem, multiple rounds of projectiles have usually been fired inbursts. Firing projectiles in bursts, however, necessitates using rapidfiring rates and also requires the use of copious amount of ammunition.In addition, the mechanical design of such a gun system becomes muchmore complex and the utility of the gun system accordingly is morelimited.

A second problem common to supercavitating projectiles is theconfiguration of the projectile itself. The primary design choiceinvolves making tradeoffs between using heavier projectiles whichgenerally result in increased range, or using lighter initially highervelocity projectiles that slow down more rapidly and are thus rangelimited. For a particular system to be effective a critical projectilespeed and mass must be selected in order to neutralize a target at agiven range.

In addition to the two projectile related difficulties described above,another obstacle encountered by rapid mine clearance systems is theobtaining of a reliable indication that the mine has in fact beendisabled. The existing approach to resolving this quandary requires theuse of scuba divers or deployment of a remote camera. In either of thesecases the mine disablement confirmation often turns into a tedious, timeconsuming process.

What is needed is a way to overcome the inherent problems associatedwith long range gun system accuracy, projectile mass and velocity designchoice considerations, and mine system clearance verificationdifficulties by providing a means for accurately dispersing multipleprojectiles at significant range while providing a reliable acousticmeans to detect target impacts thereby and either indicate an object'spresence in particular volume or lack thereof.

SUMMARY OF THE INVENTION

Accordingly, it is a general purpose and object of the present inventionto provide a system employing supercavitating projectiles operating soas to allow efficient mine clearance or platform self defense at a greatdistance from a launch platform.

It is a further object that the above general object is accomplishedusing a two stage high speed supercavitating projectile with atraditional first stage and a second stage having a plurality of smallpellet like projectiles capable of simultaneous dispersal atsupercavitation speed when the first stage projectile nears its maximumrange.

A still further object is that both the first stage projectile and theplurality of second stage pellets be entrained in vaporous cavitiesowing to the supercavitation effect of high speed motion.

Still another object is to utilize the distinctive noise signaturesproduced by suddenly collapsing vaporous cavities about the second stagepellets upon solid object impact in conjunction with an underwateracoustic targeting system to detect and track target objects.

These objects are accomplished with the present invention by providing ahigh velocity, acoustic signal producing underwater shotgun system thatdisperses a plurality of relatively small supercavitating projectilesover a wide spatial field at a long distance from the shotgun. Thesystem relies upon the dynamics of cavity collapse for better targetlocalization in underwater mine clearance operations. To reach thisobjective, a typical single stage supercavitating projectile design isenhanced to produce a two-staged projectile. The first stage of the twostage design allows for the long range firing underwater typical of asupercavitating projectile while the second stage permits the coverageof a wide area with a plurality of small supercavitating projectilesjust as the first stage projectile reaches its fixed supercavitationrange limit. A distinctive acoustic feature of the radiated noise from asupercavitating projectile impacting a solid object is used inconjunction with the two stage projectile design to provide a system forunderwater mine clearance verification. This distinctive noise signalmay also be used in conjunction with an underwater targeting system tohelp identify, localize and track targets as well.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and many of the attendantadvantages thereto will be readily appreciated as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 shows a typical prior art supercavitating projectile launchsystem;

FIG. 2 shows an embodiment of a shotgun super-cavitating projectilelaunch system built according to the teachings of the present invention;

FIG. 3 shows a comparison of the trajectories of a prior art projectileand the novel shotgun projectiles of the present invention;

FIG. 4 shows the likelihood of striking an object with both aconventional projectile and the novel pellet projectiles of the presentinvention;

FIG. 5 shows a top view of how a prior art gun system would operateagainst a number of fixed targets when the gun is fired from astationary platform; and

FIG. 6 shows a top view of how a gun system built according to theteachings of the present invention would operate against a number offixed targets when the gun is fired from a stationary platform.

DETAILED DESCRIPTION OF THE INVENTION

Accurate localization of underwater targets is a major technicalchallenge for a variety of underwater systems. Target identificationusually relies on the transmission of an acoustic signal from a fixedlocation and processing of a return echo at that same location. Thisinvention in its most basic form uses a supercavitating projectile firedfrom a fixed location to produce a radiated acoustic signal upon impactat the location of the targeted object. The radiated acoustic signaleither has greater amplitude or is easier to characterize than would bea signal transmitted from the receiver location. Previous approachesused the noise generated by a prior art projectile supercavity as atraveling noise source. This invention uses the dynamics of cavitycollapse to produce a much greater radiated noise signature and in sodoing producing much better target localization information.

A series of experiments were performed to quantify the radiated noisesignature from underwater projectiles. The experiments showed thatcavity collapse dynamics produced unique characteristic signatures. Thepresent invention modifies the basic supercavitating projectile designto greatly enhance the production of those unique characteristics.Having incorporated these features into the projectile design, a “stagedacoustic shotgun tracer bullet” has been created. This projectile maythen be used in an existing gun to augment targeting. Thus, as in thecase of an in-air tracer bullet, some of the novel projectiles may beused in a rapid-fire gun configuration to localize the proximity of thestandard projectile stream on the intended target.

FIG. 1 shows a traditional supercavitating projectile gun systemgenerally indicated as 10. A tapered cylindrical projectile 12 is housedin a cylindrical gun barrel 14. Within barrel 14 a cylindrical charge 16at the aft end of the barrel propels both projectile 12 and acylindrical metal pusher 18 forward. The metal pusher 18 allows thepressure in barrel 14 to rise in a desired manner and to accelerateprojectile 12 without any deformation in the projectile's shape. Toensure the projectile smoothly transverses barrel 14 a taperedcylindrical sabot 20 is used to fill the space between the insidediameter of barrel 14 and the tapered outer surface of projectile 12.The sabot is typically constructed of lightweight foam-like material.Sabot 20 separates and falls away upon projectile 12 water entry. Acavitator tip 22 is affixed to the front of projectile 12. Tip 22 andthe high speed transit of projectile 12 in the water environment due tothe force produced by charge 16 produces a region of water vapor aroundprojectile 12 called a vaporous cavity. The complete envelopment ofprojectile 12 within this vaporous cavity is termed “supercavitation”.

The supercavity so formed terminates in a cavity closure region aft ofthe projectile. The cavity closure region is usually well definedspatially but is not steady. Quasi-steady rupture of the cavity closureregion produces a trail of small water vapor bubbles behind the closurepoint. The bubbles in the wake ultimately collapse to produce a largeamplitude radiated acoustic signature. The entire grouping of cavitatortip 22, near stable supercavity and trailing collapsing bubble cloudadvects through the water at the speed of the cavitator tip. Theradiated signal from the supercavitating projectile 12 is verypredictable for a wide range of projectile geometries and subsonicspeeds.

A projectile's supercavity tends to be upwards of 20 feet long. Theusual major source of noise that the supercavity produces is from thecollapsing bubbles in the wake of the supercavity. The supercavityitself acts to baffle noise propagating in the forward direction in thevicinity of the projectile trajectory. This baffling of the forwardpropagating sound and the time delay associated with the cavity lengthexplains the main features of the typical projectile's radiated noisesignature.

When a projectile strikes an object however, the cavity collapsecontinues to proceed from well aft of the projectile strike toward theimpact location. Note that a premature cavity collapse produces acousticnoise that is significantly higher in amplitude than a non-impactcollapse. Thus, if a supercavitating projectile strikes an object, acharacteristic pulse of approximately 5 ms duration is observed at theimpact location. The time delay in when that pulse is observed is afunction of supercavity length and projectile speed. Similar signalshave been associated with different sized projectiles including smallpellet-like projectiles.

The preferred embodiment of the present inventive system, generallyidentified as 100, is shown in FIG. 2. System 100 includes a gun barrel14, a propellant charge 16, a projectile further generally identified as102, a pusher 104 and sabots 20. Projectile 102 includes an elongatedcylindrical structure 106 having a concave surface 106 a, a combustionchamber 108 at the forward end of structure 106 containing a smallexplosive charge, a cavitator tip 22 forward of chamber 108, a convexouter foam cover 110 extending from the aft end of structure 106 to theforward end of tip 22, a core region fuse 112 passing through a centralaperture 114 in structure 106 and into chamber 108, an interior volume116 created between the concave surface 106 a of structure 106 and theinterior surface of convex cover 110, and a plurality of pellets 118filling interior volume 116. Core region fuse 112 is constructed of aflammable material. Fuse 112 also passes through an aperture 120 inpusher 104 and into charge 16. Upon launch command the flammablepropellant charge 16 in barrel 14 is ignited. Combustion of thepropellant charge 16 further causes a flame front to move in a timedmanner along fuse 112 from the projectile aft section through aperture120 in pusher 104 and on through aperture 114 in concave structure 106into combustion chamber 108. As the flame front reaches small combustionchamber 108 near the end of the projectile's expected travel, a smallexplosion occurs. The explosion causes cavitator tip 22 to separate fromprojectile 102 and permits the external water flow to contact convexfoam cover 110. Foam cover 110 is ripped away from the projectile by thewater exposing interior volume 116. Interior volume 116 contains thelarge number of shotgun-like pellets 118, which then disperse over awide area. These pellets can be similar to conventional shotgun pellets.Due to the high initial pellet velocity at the time of cavitator tip 22ejection, a small supercavity will envelop each pellet. The rate offlame front propagation from charge 16 to combustion chamber 108 willdetermine the range from the gun barrel where pellet separation takesplace. The overall design of the shotgun projectile is shown to fitsubstantially within the same gun barrel volume as did the prior artprojectile.

FIG. 3 shows a side view comparison of the trajectories 140 and 142 ofthe prior art projectile 12 and the instant invention projectile 102respectively. An underwater gun 144 launches a prior art projectile 12along trajectory 140 through water medium 146. The prior art projectilefollows trajectory 140 for a distance R at which point the projectilespeed has slowed sufficiently to cause the supercavity to collapse and arapid slowing of the projectile ensues along a tumbling trajectory 148,the projectile falling until it reaches ocean floor 150. Note thisbehavior does not produce a high amplitude noise. In the case of thepreferred embodiment projectile 102, before the critical slow down rangeR′ is reached the small explosion occurs at the point designated as 152.At that point multiple trajectories 154 develop and each terminate aftera relatively short distance (approximately (R-R′)). It is noteworthythat the total range of the device of the instant invention is slightlyshorter than that of the prior art device. The prior art device acts tooptimize range for a fixed mass projectile while the instant inventionact to optimize target detection.

FIG. 4 shows a comparison of the likelihood of striking an object usingeach of the projectiles of FIG. 3. The probability trace 160 for theprior art projectile along the entire trajectory is associated with theprojected area of the projectile. The instant invention casedemonstrates the greatly expanded likelihood of hitting an object nearrange R′ as shown by trace 162.

FIG. 5 shows how a prior art gun system would operate against a numberof fixed targets when fired from a platform 200 housing gun barrel 144and a sensor 202 and FIG. 6 shows how an instant invention gun systemwould operate against a number of fixed targets when fired from aplatform 200 housing gun barrel 144 and a sensor 202. In the prior artcase a spray pattern including individual trajectories 204 would beattempted to try to strike targets 206. As the range of targets 206increases, the likelihood of success diminishes. Also, without apreprogrammed search for the telltale 5 ms return from an impact cavitycollapse there is no indication of success. In the case of FIG. 6 anumber of projectiles designed for slightly different ranges are firedalong trajectories 204 i, 204 i+1 and 204 i+2. Pellet spread patterns208 provide wide coverage volumes at each distance from gun barrel 144.This results in a small number of projectiles covering a large areaalong and across trajectory 204. The present inventive system is muchmore efficient in terms of total projectiles expended if knowledge ofobject range is available. The problem of a small error in a rangetargeting system has little impact on the implementation of this system.In addition the instant invention system contains a sonar system 210that receives the sensed data from sensor 202 and searches for targethit returns 212, adjusting the ensuing projectile firing trajectories inresponse.

The primary advantages of the acoustic shotgun system are many andvaried including: the ability to produce a pattern of supercavitatingprojectiles over a wide area from a single round; the ability to controlthe dispersion area; the ability to control the range over which thisdispersion occurs; the ability to accurately determine projectile missesand hits; the means to produce a targeting solution and firing thatminimizes the effects of single projectile dispersion; the use of cavitycollapse upon impact to produce radiated projectile noise; the abilityto operate in an existing gun system; improved ability to cover a largervolume with supercavitating projectiles; improved determination of thetrack of an underwater object; the ability to detect supercavitatingprojectile impacts; better resolution of underwater objects and tracksin poor acoustic environments; more efficient mine clearance operation;better ability to engage multiple targets; and, more efficient use ofprojectile volume and hence improved overall gun system efficiency.

What has thus been described is a system of supercavitating projectilesand operating procedures that enable efficient mine clearance orplatform self defense at great distance from a launch platform. This isaccomplished with the present invention by providing a high velocity,acoustic signal producing underwater shotgun system that disperses aplurality of relatively small supercavitating projectiles over a widespatial field at a long distance from the shotgun. The system reliesupon the dynamics of cavity collapse for better target localization inunderwater mine clearance operations. To reach this objective, a typicalsingle stage supercavitating projectile design is enhanced to produce atwo-staged projectile. The first stage of the two stage design allowsfor the long range firing underwater typical of a supercavitatingprojectile while the second stage permits the coverage of a wide areawith a plurality of small supercavitating projectiles just as the firststage projectile reaches its fixed supercavitation range limit. Adistinctive acoustic feature of the radiated noise from asupercavitating projectile impacting a solid object is used inconjunction with the two stage projectile design to provide a system forunderwater mine clearance verification. This distinctive noise signalmay also be used in conjunction with an underwater targeting system tohelp identify, localize and track targets as well.

Obviously many modifications and variations of the present invention maybecome apparent in light of the above teachings. A number of alternativedevices could be constructed using the same general methods discussedherein to construct devices that would be optimized for a particularpurpose. For example: the projectile shape and size is not limited asshown; the teachings of this invention may be used to design projectileswith different ranges, calibers, and different pellet patterns; anynumber of pellet shapes can be used; the projectile structural coredesign is not unique, alternative cover materials and shapes could beused to enclose the shotgun pellets within the projectile; multipleprojectiles could be fired from a Gatling gun analogous to tracerbullets in addition to prior art projectiles to provide a nearcontinuous closed loop targeting system; the projectiles could bedesigned with any number of enclosed pellets and different size enclosedvolumes to produce different dispersion patterns; and, differentflammable core materials could be used.

In light of the above, it is therefore understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

1. An acoustic shotgun system comprising: a tapered underwaterprojectile capable of operating at supercavitation speed, saidsupercavitating projectile having disposed therein a plurality ofpellets; a cylindrical gun barrel for holding and firing said underwaterprojectile, said gun barrel having an inside diameter sized toaccommodate said tapered underwater projectile; a sensing meanspositioned underwater adjacent said gun barrel, for detecting andtransmitting said acoustic signal from said pellets; a sonar systemattached to said sensing means, capable of receiving said transmittedacoustic signal from said sensing means and determining therefrom therange and position of a target; a cylindrical pusher, disposable in saidgun barrel aft of said projectile in contact therewith, for exerting amotive force upon said projectile sufficient to produce supercavitationthereof; a first explosive charge, positioned aft of said pusher, forcontrollably exerting upon detonation said motive force against saidpusher; and a tapered foam sabot, surrounding the tapered portion ofsaid projectile in said gun barrel, for providing moveable support tosaid projectile during launch.
 2. The shotgun system of claim 1, whereinsaid pusher has a central aperture passing therethrough from forward toaft and wherein said projectile further comprises: a concave taperedcore structure, said taper running aft to forward, said core structurefurther having a central aperture passing therethrough from forward toaft; a combustion chamber, fixedly attached to the forward end of saidcore structure; a second explosive charge housed in said combustionchamber; a convex foam cover disposed over said core structure and saidcombustion chamber, for creating an interior volume therebetween tohouse said plurality of pellets; a cavitator tip fixedly attached at theforward extreme of said core structure, said combustion chamber and saidfoam cover; and a fuse disposed between said first explosive charge atthe aft end of said barrel and said second explosive charge in saidcombustion chamber, said fuse passing through said pusher centralaperture and said tapered core structure central aperture such that uponignition of said first explosive charge said fuse lights andcontrollably propagates a launch timing flame front therethrough intosaid combustion chamber igniting said second explosive charge fordispersing said plurality of pellets from said projectile at apreselected time after launch.
 3. The shotgun system of claim 2, whereinsaid plurality of pellets are of uniform size.
 4. The shotgun system ofclaim 3, wherein said acoustic signal sensing means is a hydrophone. 5.The shotgun system of claim 4, wherein said cylindrical gun barrel isdisposed underwater.
 6. The shotgun system of claim 4, wherein saidcylindrical gun barrel is disposed on a surface platform.
 7. A methodfor localizing the range and bearing of a distant underwater object, themethod comprising the steps of: firing a preselected number ofsupercavitating projectiles sequentially from a gun with each projectiletracking along substantially the same trajectory, each projectilefurther being preset to disperse a plurality of supercavitating pelletsstored therein over a wide volume pattern that spreads orthogonal tosaid trajectory at a unique, pre-selected range from said gun, theplurality of volume patterns thus produced by said correspondingsupercavitating projectile further being closely spaced along saidtrajectory, whereby a small number of projectiles thus serve to cover alarge volume along and across said trajectory; sensing acoustic signalsproduced by supercavitating pellet impact with said distant object usingan acoustic sensor located proximate said gun, said pellet impactproducing a detectable characteristic bubble collapse signature returnof higher amplitude and distinctive pulse length that is easier todetect than a standard sonar echo; and processing said acoustic signalreturn data using a signal processing sonar system connected to saidacoustic sensor to search for object range and bearing data from saidbubble collapse returns.
 8. The method according to claim 7, whereineach said bubble collapse produces a distinctive acoustic noise pulsereturn of 5 ms duration.
 9. The method according to claim 8, whereineach said multi-pellet projectile is fired as part of a stream ofstandard supercavitating projectiles for use as a tracer bullet indirecting the flow of standard projectiles upon said target by adjustingensuing projectile firing trajectories in response to said object rangeand bearing location data.